Steels with high manganese contents, due to their advantageous characteristic combination of high strength of up to 1,400 MPa on the one hand and extremely high elongations (uniform elongations up to 70% and breaking elongations up to 90%) on the other hand, are basically suitable to a special degree for use in the vehicle manufacturing industry, in particular automotive construction. Steels, particularly suitable for this specific application, with high Mn-contents of 7% by weight up to 30% by weight are known for example from the German documents DE 102 59 230 A1, DE 197 27 759 C2 or DE 199 00 199 A1. Flat products fabricated from these steels have isotropic deformation behaviour with high strength and are also ductile at low temperature.
However, counteracting these advantages, steels with higher Mn-contents are susceptible to pitting corrosion and can only be passivated with difficulty. This high propensity, compared to lower alloyed steels, to locally limited but intensive corrosion in the presence of increased chloride ion concentrations makes it difficult to use steels belonging to the material group of highly alloyed sheet steel. In addition, high manganiferous steels are susceptible to surface corrosion, which likewise limits the spectrum of their use.
Therefore, it has been proposed to also provide flat steel products, which are fabricated from high manganiferous steels, with a metallic coating in the way known per se, which protects the steel against corrosive attack. For this purpose, attempts have been made to apply a zinc coating to the steel material electrolytically. Although these attempts have led to zinc-coated strips, whose base material in each case can consist of the highly alloyed steel, which is protected against corrosion by the metallic coating applied thereto, it has been shown that when such sheet metal is welded, problems occur in the area of the welding zone, which are known in the industry under the term “soldering brittleness”. As a consequence of welding the grain boundaries in the base metal are infiltrated by liquefied zinc material of the coating. This infiltration causes the material in the surrounding area of the welding zone to lose strength and ductility to such an extent that the welded joint and/or the sheet metal bordering the welded joint no longer fully meets the requirements for stability under load.
Attempts to produce sheet metal which is sufficiently corrosive-resistant and at the same time has good welding properties by the known hot-dip coating methods, remain unsuccessful, because highly alloyed steel strips, in particular such with Mn-contents of more than 6% by weight, after annealing which is essential for the effectiveness of the hot-dip metal coating, are susceptible to heavy surface oxidation. The sheet metal surfaces, oxidized in such a manner, can no longer be wetted by the metallic coating to the necessary degree of uniformity and entirety, so that the aim of total surface area corrosion protection cannot be achieved in this way.
The possibilities, known from the spectrum of steels being highly alloyed but having lower Mn-contents, for improving wettability by applying an intermediate layer of Fe or Ni in the case of steel sheets comprising at least 6% by weight manganese have not led to the desired success.